create (and pass to sessions) a "temporary directory" they can use for output files

[moshez]

How would this feature be useful?

When testing documentation builds, it is nice to have the output be under the nox temporary directory instead of in an OS-specific temporary directory. It makes cleanup easier, and it means there is a consistent URL to be loaded in the browser.

Describe the solution you'd like

A session should have access to a variable "environment temporary directory", and pass it on to commands as appropriate (for example, as the -d option to sphinx).

Describe alternatives you've considered

In theory, session could grab the virtual environment directory and create a tmp subdirectory there themselves, but this is a little ugly.